1. Field of the Invention
This invention is relates to a mobile communication system and a mobile communication terminal, and more particularly, suitably applied to a digital cellular system in a code division multiple access (CDMA) system.
2. Description of the Related Art
A digital cellular system is comprised of a mobile terminal and plural base stations, and constructs a so-called wide area communication system. The mobile terminal radios with the plural base stations, thereby communicating with a public telephone or another mobile telephone in the form of a voice or the like, via a wire circuit or a radio circuit which is connected with the base station.
The plural base stations transmit and receive radio waves under a predetermined radio communication system, and mediate between call-out from the mobile terminal and call-in to the mobile terminal. Besides, the base station periodically transmits its own identification signal so that the mobile terminal can recognize the base station, and moreover periodically transmits an adjacentstation signal which is an identification signal of the adjacent base station.
That is, the mobile terminal receives the adjacent-station signal together with the identification signal of the base station by receiving signals sent from the base station while moving. Since the reception sensibility of a radio wave lowers as the mobile terminal departs from the service area of the base station, the mobile terminal examines the reception sensibility of the radio wave of the adjacent base station which is included in the adjacent-station signal, in preparation for hand-off. As a result, when the radio wave of the adjacent base station has become so strong as to be received satisfactorily, the mobile terminal performs hand-off toward the adjacent base station, so that it can continue talking while moving.
By the way, the mobile terminal having the aforementioned structure can continue talking while moving at high speed by performing hand-off among plural base stations while moving. A global positioning system (GPS), one of various known navigation systems, to obtain positional information by utilizing radio waves which are emitted from an artificial satellite is used as a system for determining the absolute position of the mobile terminal at that time. In this case, by combining the GPS with the mobile terminal, it is enabled to determine the absolute position of the mobile terminal. However, since this case requires to install an antenna and a receiver in addition, there have been problems in that the system is enlarged in size as a whole and so it has poor handiness.
On the other hand, since the simple-type mobile telephone system called a personal handyphone system (PHS) has a small service area of about 100 [m] to 300 [m] in comparison with the fact that the service area of the mobile telephone system is about 1.5 [km] to several kilometers, if the PHS base station side gives notice of the positional information of the PHS base station which is communicating with the mobile terminal, the mobile terminal side can know that to which PHS base station it is connected. However, there has been such a problem that though the user can recognize that his mobile terminal is staying within the small service area of the PHS base station, the recognition has an error outside of about 100 [m] to 300 [m].
Besides, there have been problems in that the mobile terminal does not have the real-time paging function for performing transmission of letter information, etc. among the mobile terminals and transmission of a response signal from the side which has received the letter information, and is not able to perform two-way communications.